hobbing helical gear

Hi. I agree with Andy, don't think this method will produce an accurate helical gear.

Think of it this way....

If you cut a radial groove on a round disc using a normal 4th axis but you keep the 4th axis stationary, using the x and y axis to produce the angle, the groove will not have a constant depth measured from the OD. Neither will it be truly radial across the width of the disc.

Alternatively, if you produce the slot using the X axis and the 4th axis in a synchronised move, you will end up with a helical slot. This slot will be truly radial and have the same depth at any section of the disc across its width.

The method above will not work IMHO. The error may be negligible at small helix angles, but a pinion with a small amount of theeth and a large helix angle of say 30 deg will show it clearly.

Henk

Or maybe not....I'm not so sure anymore...have to think about this a bit more
Henk

Hi. Yes, we do helical and worm gears on our pfauter. Worm gears can be done with either radial feed or tangential feed. We have cut some worm gears using a fly hob as well. This is basically a single tooth fly cutter for worm gears

One more interesting video showing helical hobbing by means of electronic gearing.

And, again, the guy accounted for hob pitch angle by twisting the spindle (so hob vanes are vertical) and accounted for gear helix angle by twisting work piece.

Hi, Henk!

I'm afraid I haven’t understood your 4th axis example. I agree with you that the slot will not be truly radial and won’t have constant depth. But I don’t think it is the case for the video.

But I also share your concerns regarding smaller gears with large helix angles. If something is wrong it will show up at the extremes.

Hello,
as I see it there are three aproaches here, that seem to have the same relative movement between hob and gear blank.
a) LSCAD he refers to hobbing by Grant method.
b) AndysMachines has basically the same setup, but in his case the complete setup is tilted 90° (hob almost vertical)
c) What I tried see picture of my earlier post. The gear blank is horizontal but I move the hob diagonally while feeding. (because my gear blank axis cannot be tilted) In all cases the spindle is tilted a little by the helix angle of the hob.
The relative movement of the hob is in all three cases (unless I overlooked something ) parallel to the tooth flank.
Pfauter keeps the axis of the gear blank constant and the machine feeds parallel to the cut.
The diagonal travel within the gear tooth is provided by additional (substraction) rotation of the gear blank.
For the a) to c) approach only a very limited length of gear tooth can be cut. As andypugh stated "with a long gear you might run out of hob length"
Using the additional rotation of the hob (Pfauter/andypugh). Gear blanks can be stacked and all cut in one operation.

The "complicated math additional rotation approach" has advantages.
1. stacking gears is easier
2. making a stiff machine will be probably easier.
3. shorter hobs can be used (makes them cheaper)